1. Field of the Invention
The present invention is directed to a method of etching anodic foil for electrolytic capacitors and more particularly, to a method of growing a porous oxide mask on a surface of a high purity etchable strip of anodic foil for forming etch tunnels at strategic locations on the foil.
2. Related Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an implantable cardioverter defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Electrolytic capacitors are used in ICDs because they have the most nearly ideal properties in terms of size and ability to withstand relatively high voltage.
Conventionally, an electrolytic capacitor includes an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, composing a planar, layered, stack structure of electrode materials with separators interposed therebetween.
Since these capacitors must typically store approximately 30-40 joules, their size can be relatively large, and it is difficult to package them in a small implantable device. Currently available ICDs are relatively large (over 44 cubic centimeters (cc)), generally rectangular devices about 12-16 millimeters (mm) thick. A patient who has a device implanted may often be bothered by the presence of the large object in his or her pectoral region. Furthermore, the generally rectangular shape can in some instances lead to pocket erosion at the somewhat curved corners of the device. For the comfort of the patient, it is desirable to make smaller and more rounded ICDs. The size and configuration of the capacitors has been a major stumbling block in achieving this goal.
In ICDs, as in other applications where space is a critical design element, it is desirable to use capacitors with the greatest possible capacitance per unit volume. Since the capacitance of an electrolytic capacitor increases with the surface area of its electrodes, increasing the surface area of the aluminum anode foil results in increased capacitance per unit volume of the electrolytic capacitor. By electrolytically etching aluminum foils, an enlargement of a surface area of the foil will occur. As a result of this enlargement of the surface area, electrolytic capacitors, which are manufactured with the etched foils, can obtain a given capacity with a smaller volume than an electrolytic capacitor, which utilizes a foil with an unetched surface.
In a conventional electrolytic etching process, surface area of the foil is increased by removing portions of the aluminum foil to create etch tunnels. The foil used for such etching is typically an etchable aluminum strip of high cubicity. High cubicity in the present context is where at least 85% of crystalline aluminum structure is oriented in a normal position (i.e., a (1,0,0) orientation) relative to the surface of the foil. The foil used for etching is also preferably of high purity. Such foils are well-known in the art and are readily available from commercial sources.
The ideal etching structure is a pure tunnel-like etching with defined and uniform tunnel diameters and without any undesirable pitting of the foil. As tunnel density (i.e., the number of tunnels per square centimeter) is increased, a corresponding enlargement of the overall surface area will occur. Larger surface area results in higher overall capacitance.
U.S. Pat. No. 4,213,835 to Fickelscher discloses a method for electrolytically etching a recrystallized aluminum foil which allows manufacture of foils with exclusively pure cylindrical or cubical etching structures and tunnel densities greater than 10.sup.7 /cm.sup.2 with an avoidance of irregular pitting of the foil. The method consists of providing an etching bath containing chloride ions, positioning the foil in the bath and potentistatically etching the foil with a temporally constant anode potential. The preferred etching step occurs in two stages. In the first stage, the etching current density is set above the potential or current density which creates pitting of the aluminum. After an induction period of around 10 seconds, the etching tunnels grow autocatalytically at a rate of several .mu.m/s with a pore diameterof approximately 0.2 .mu.m in the crystal oriented direction (i.e., a (1,0,0) orientation relative to the surface of the foil). After approximately one minute of exclusive tunnel formation and in order to avoid the occurrence of coarse pitting, the etching current density is reduced. In the second stage, the current density is set below the current density which creates pitting of the aluminum, such that only pore or tunnel enlargement up to the desired value will occur. Thus, the etching time for the tunnel enlargement is relatively long in relation to the etching time for obtaining the tunnel structure in the foil.
U.S. Pat. No. 4,420,367 to Locher discloses a similar method for etching aluminum foil for electrolytic capacitors. Electrolytic tunnel formation is carried out in a first etching stage, as described above. However, the further etching for tunnel enlargement is non-electrolytic, taking place chemically in one or several etching stages. The method is preferably carried out in a halogen-free or chloride-free solution having nitrate ions, such as HNO.sub.3 and/or Al(NO.sub.3).sub.3.
U.S. Pat. Nos. 4,474,657, 4,518,471 and 4,525,249 to Arora disclose the etching of aluminum electrolytic capacitor foil by passing the foil through an electrolyte bath. The preferred bath contains 3% hydrochloric acid and 1% aluminum as aluminum chloride. The etching is carried out under a direct current (DC) and at a temperature of 75.degree. C. U.S. Pat. No. 4,474,657 is limited to the above single step. U.S. Pat. No. 4,518,471 adds a second step where the etched foil is treated in a similar bath with a lower current density and at a temperature of 80-82.5.degree. C. U.S. Pat. No. 4,525,249 adds a different second step, where the etched foil is treated in a bath of 8% nitric acid and 2.6% aluminum as a nitrate, at a temperature of 85.degree. C.
In the first or tunnel forming stage of the above methods for etching aluminum foil for electrolytic capacitors, the number and distribution of the etch tunnels is related to the magnitude of the applied current density and the dissolved amount of the foil is approximately in proportion to the quantity of the electricity applied. Therefore, in order to increase the specific capacitance of the aluminum foil, the current density is set large to form dense tunnels and the quantity of applied electricity is increased to increase the effective surface gain of the foil.
As the etching progresses, the density of the tunnels increases gradually and the unetched part of the foil surface decreases gradually. However, in a conventional tunnel formation step, where the current is applied to the foil under constant conditions for the applied time, tunnel formation is not consistent across the foil surface. As such, formed tunnels are etched excessively while new tunnels are formed, causing portions of the etched foil surface to come off. U.S. Pat. No. 5,503,718 to Kakizakai discloses a two stage method of etching aluminum foil for electrolytic capacitors, as described above, in which the electric current applied during the step of tunnel formation is decreased from a maximum value continuously or stepwise so that the current density for the unetched part is kept approximately constant with the course of time. In this way, the etched surface can be prevented from coming off as the unetched surface is etched and the capacitance per etched amount is improved.
In other known etch processes, the anode foil is pretreated (treated prior to etching) in order to maximize the increase in surface area and improve the distribution of etch tunnels during the subsequent etching steps. A mechanical pretreatment can be applied, such as stroking the surface of the foil with a high speed rotating metal brush to remove a surface layer and uniformly texture the surface of the foil. A chemical pretreatment, such as commercial cleansing agents, acid solutions, or alkaline solutions, can be applied to remove the residual processing oils form the surface of the metal foil and dissolve surface oxides, or replace surface oxides with a new surface film. Alternatively, an electrochemical pretreatment, as disclosed in U.S. Pat. Nos. 4,437,955 to Shaffer and 4,676,879 to Salvadori can be applied to remove a relatively small amount of the surface metal during an initial etch step, as compared to the amount of surface metal removed during the subsequent primary etch step.
A pretreatment process can also consist of depositing a metal film onto the foil surface prior to etching the foil in order to enhance the resulting capacitance of the foil. For example, U.S. Pat. No. 5,405,493 to Goad discloses a method for etching aluminum anode foil in which the foil is pretreated by depositing a discontinuous surface layer of metal that is cathodic to the foil, followed by chemically etching the foil to remove a portion of the deposited metal. Finally, the foil is etched to create the etch tunnels. The discontinuous metal layer, deposited during the first pretreatment step, and the aluminum surfaces exposed by the chemical etching of the second pretreatment step, act as local sites for cathodic reactions during the final etch step, creating a substantial number of etch tunnels near the deposited metal cluster sites.
However, the above disclosed methods, which maintain adequate metal strength and improve capacitance, are not sufficient to produce a foil capable of yielding the very high energy densities (&gt;6 Joule/cm.sup.3) needed for advanced ICD designs. There is a need for an improved method for etching anode foil in which tunnel density is increased over conventional etching methods.